It's generally not a required part of a stage manager's job to check that none of the actors are drunk before the curtain goes up, which is why it comes as a complete shock to Philip when Heidi's policewoman stumbles onto the stage at the top of the second act, shirt half buttoned.
It's not immediately apparent that something is wrong, but within minutes they've skipped two pages and she's giggling at every awkward pause. The other actors, to their credit, manage to roll with it - Philip can only sit and watch the trainwreck unfold, listening out for the next mangled cue line.

Heidi's fled by the time the house lights come up. Over drinks that evening, the shell-shocked cast's reactions range from barely-suppressed rage to a resigned disappointment. Philip sits outside to keep the SM company; she chain smokes while they put off calling the director, trade war stories, and lament the fact that you have to experience it in order to have it to tell.